The present invention relates to a process for preparing chlororhodiumic acid by dissolving rhodium in hydrochloric acid.
Heretofore, in order to prepare chlororhodiumic acid (H.sub.3 RhCl.sub.6), the mixture of rhodium powders and sodium chloride or barium chloride is heated in a chlorine gas atmosphere so that the rhodium powders are converted into such a soluble salt as Na.sub.3 RhCl.sub.6 which is extracted with water to provide a chlororhodiumic acid solution. Since, however, sodium or barium is contained in this solution, this method has the disadvantages that Na.sup.+ or Ba.sup.2+ should be removed by means of ion exchange or the like, and that the soluble salt is difficult to be formed so that the yield becomes low.
In addition to this method, various researches have been performed in which rhodium powders and chlorine are reacted to produce rhodium chloride. However, in all the researches, since a temperature higher than 250.degree. C. is required for the said reaction, an apparatus having the thermostability and the like is required so that the economical operation cannot be assured when the reaction is employed as one of the processes for the production of the chlororhodiumic acid.
Further, although fine rhodium black which is slightly dissolved in concentrated hydrochloric acid provides chlororhodiumic acid after being boiled with the hydrochloric acid, the dissolved amount thereof is insufficient so that the economical operation cannot be performed.
Although a method for extracting platinum group metals such as rhodium supported on such an inorganic porous support as activated alumina or the like employing hydrochloric acid and an oxidizing agent is also proposed, the yield thereof cannot be raised because the metals to be extracted is supported, so that the washing process is further employed after the extraction by the hydrochloric acid and the oxidizing agent for improving the recovery rate. By employing this method, a satisfactory recovery rate can be attained, but it is desirable, for improving the operation efficiency, to reduce the number of the processes employed. Further, this method cannot be applied without modification to the production of the chlororhodiumic acid.